1. Field of the Invention
This invention relates to a system for and a method of detecting or estimating the remaining charge of an electrical double layer capacitor.
2. Description of the Related Art
Various types of high-capacity electrical double layer capacitors composed of a combination of activated carbon or the like and an electrolyte have recently been developed and applied as, for instance, energy sources of hybrid vehicles., as taught, for example, by Japanese Laid-open Patent Application No. Hei 11(1999)-220810.
When an electrical double layer capacitor is used in such an application, its remaining charge must be accurately detected. The conventional technique used to detect the remaining charge has been to integrate the charge/discharge current detected by a current-voltage sensor and correct the result for the internal resistance.
When the remaining charge is detected chiefly by current integration, however, a high-precision current sensor must be utilized to achieve high detection accuracy. This makes the cost high. Since the error in the estimated remaining charge grows larger with degradation of the current sensor, moreover, the inconvenience arises of having to initialize or correct the calculated value when the electrical double layer capacitor is fully charged or has a prescribed remaining charge.
The present invention aims to overcome the foregoing problems and has as an object to provide a system for and method of detecting or estimating the remaining charge of an electrical double layer capacitor that uses a mathematical model having the remaining charge of the electrical double layer capacitor as a state variable to calculate or estimate the remaining charge of the electrical double layer capacitor from its detected terminal voltage and charge/discharge current, thereby enabling simple and accurate detection of the remaining charge of the electrical double layer capacitor without need for a high-precision current sensor.
The present invention achieves the foregoing object by providing a system for detecting remaining charge of an electric double layer capacitor, comprising: a current-voltage sensor which generates an output indicative of a terminal voltage across terminals of the capacitor and an output indicative of charge/discharge current charged into and discharged from the capacitor; and an electronic control unit which inputs the outputs of the current-voltage sensor indicative of the terminal voltage and charge/discharge current and calculates a remaining charge of the capacitor based on a mathematical model having the remaining charge of the capacitor as a state variable.